linux-kernel - Re: [REGRESSION] x86_32 boot hang in 6.19-rc7 caused by b505f1944535 ("x86/kfence: avoid writing L1TF-vulnerable PTEs")

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Message-Id: <20260126122440.78e7ffebd5257e5ce00fa35a@linux-foundation.org>
Date: Mon, 26 Jan 2026 12:24:40 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Ryusuke Konishi <konishi.ryusuke@...il.com>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>, Marco Elver
 <elver@...gle.com>, LKML <linux-kernel@...r.kernel.org>, Alexander
 Potapenko <glider@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Thomas
 Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav
 Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML
 <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Jann Horn
 <jannh@...gle.com>, kasan-dev@...glegroups.com, stable
 <stable@...r.kernel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [REGRESSION] x86_32 boot hang in 6.19-rc7 caused by
 b505f1944535 ("x86/kfence: avoid writing L1TF-vulnerable PTEs")

On Tue, 27 Jan 2026 04:07:04 +0900 Ryusuke Konishi <konishi.ryusuke@...il.com> wrote:

> Hi All,
> 
> I am reporting a boot regression in v6.19-rc7 on an x86_32
> environment. The kernel hangs immediately after "Booting the kernel"
> and does not produce any early console output.
> 
> A git bisect identified the following commit as the first bad commit:
> b505f1944535 ("x86/kfence: avoid writing L1TF-vulnerable PTEs")

Thanks.  b505f1944535 had cc:stable so let's add some cc's to alert
-stable maintainers.

I see that b505f1944535 prevented a Xen warning, but did it have any
other runtime effects?  If not, a prompt revert may be the way to
proceed for now.

> Environment and Config:
> - Guest Arch: x86_32  (one of my test VMs)
> - Memory Config: # CONFIG_X86_PAE is not set
> - KFENCE Config: CONFIG_KFENCE=y
> - Host/Hypervisor: x86_64 host running KVM
> 
> The system fails to boot at a very early stage. I have confirmed that
> reverting commit b505f1944535 on top of v6.19-rc7 completely resolves
> the issue, and the kernel boots normally.
> 
> Could you please verify if this change is compatible with x86_32
> (non-PAE) configurations?
> I am happy to provide my full .config or test any potential fixes.
> 
> Best regards,
> Ryusuke Konishi